I took a floral design class in my first semester of college. Probably the best class I've had, seeing as I've gotten to arrange a lot of plant stuff as of late. So, with a long tank, a really cool thing to do is keep a big open area in the middle with some bulky center pieces, but at the edges you'll want to gradually lift the eye-line upwards with plants. Here's an example of what I mean by that. See how it gradually lifts? You can use the same plant, or different ones.

I've found that throwing in a complementary color for rocks usually helps bring out your betta's colors more and makes them more visually appealing (as if they aren't already, right?). Don't forget your floater plants!!!

Dude that looks so much better than mine. more detail in your plants...

Lol, thanks. I really liked your plant style, though! I couldn't get my paint to do that cool see-through look successfully. It kept making the background all dotted and funky. >.< That's what I was really wanting to do. :( I've never done a real paint art thing before, so that's my first one. o_o;